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Author Topic: The sun rises from the east and sets in the west, but does it?  (Read 5682 times)

paul.fr

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Is it exactly east and west, or a few degrees eather side? Is it slightly different each day?


 

Offline DoctorBeaver

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It varies. Starting from the winter solstice (Dec 21-22nd) the sun rises & sets slightly further north each day (in the northern hemisphere) up until the summer solstice (June 21-22nd) from which time it heads south until the winter solstice, then it all starts again.

An interesting point about this is that on midsummer day the sun rises diametrically opposite the point where it sets at the winter solstice, and sets diametrically opposite the point where the sun rises on the winter solstice.

It is now thought that Stonehenge, rather than being the site of celebrations for the summer solstice, was actually made aligned to the rising & setting of the sun at the winter solstice.

I think there's a lot of validity in that as it makes more sense that the ancients would celebrate the start of the sun's passage northwards, heralding the longer days, rather than celebrating the start of shorter days at the summer solstice.
« Last Edit: 02/10/2007 07:23:03 by DoctorBeaver »
 

lyner

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www.definity-systems.net/~apw/astro/sun.html
There are some pretty pictures here.
The string of beads picture showing the Sun's position over the year is stunning.
The variation of 'noon' over the year, due to the elliptical orbit is something hard to describe / explain without waving your arms around.
Quote
An interesting point about this is that on midsummer day the sun rises diametrically opposite the point where it sets at the winter solstice, and sets diametrically opposite the point where the sun rises on the winter solstice.
That is a heartening thought in the depths of winter!
The difference between a sidereal day and a solar day  is something that people don't always think of. I remember giving a Science lesson in which a girl pupil did pirouettes around the room, counting the number of times she saw the window and how many times she saw the middle of the room. Very graphic, but I was concerned she might get giddy and fall over. Was my risk assessment adequate? I think some of them 'got it'.
 

Offline DoctorBeaver

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I don't think we'll go into the precession of the equinox.  ;D
 

Offline techmind

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Is it exactly east and west, or a few degrees eather side? Is it slightly different each day?
To a very good first approximation, and for an arbitrary latitude on the earth, the sun rises due east, and sets due west only around the time of the spring or autumn equinox (March 22nd and Sept 22nd). On the equator the sun will rise in the east and set in the west throughout the whole year. Under these conditions the sun will be above the horizon for 12 hours a day.

For other times of the year (and not on/near the equator) days will be longer in the summer (in the northern hemisphere: sun rises north of due east in the northern hemisphere, and sets north of due west), and shorter in the winter (rises south of east, sets south of west).

As an example, in London we get as little as 8 hours (one-third of a day) sunshine in December, or as much as 16 hours (2/3 day) in June. Taking those as fractions of 360degrees, I reckon the sun should rise/set as much as 30 degrees north of east/west (summer) or south of east/west (winter). For Scotland the variation in length of day is even more extreme.

This is the basic principle, based on the earth spinning on a tilting axis as it orbits the sun. There are minor variations to this owing to 'wobbles' in the orbit and spin-axis. A web search for "equation of time" will give plenty of related information on some of the details.
 

Offline DoctorBeaver

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Techmind - that's not strictly accurate. I used to live in Uganda and had the equator running through my back garden. As the sun's zenith travels from the tropic of Cancer to the tropic of Capricorn and back again, that means there are 2 longest days - once when the sun crosses the equator heading north, and the other as it crosses the equator heading south again.

Although this is true of all latitudes between the tropics, near to the tropics it is barely noticeable and is at its most pronounced on the equator.
 

Offline techmind

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Techmind - that's not strictly accurate. I used to live in Uganda and had the equator running through my back garden. As the sun's zenith travels from the tropic of Cancer to the tropic of Capricorn and back again, that means there are 2 longest days - once when the sun crosses the equator heading north, and the other as it crosses the equator heading south again.
it is barely noticeable and is at its most pronounced on the equator.
I admit I was a bit shakey on that when I wrote it, trying to visualise things late at night etc. I can't argue with the facts, and stand corrected... or not? I can now visualise that on the equator the sun can rise/set north/south of due east/west (other than at the equinox)... but I don't quite see how the day-length can deviate from 12 hours. Need to find a reference which does all the maths properly!

How much difference is there between the longest and shortest days on the equator then? I'm guessing it's not more than a few tens of minutes...?
« Last Edit: 03/10/2007 15:00:05 by techmind »
 

Offline DoctorBeaver

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You're right, it's only a few minutes. If I remember rightly, the max difference is about half an hour.
 

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